Correcting cross-track errors in a linear printhead

1. A method for correcting cross-track position errors in a digital printing system having a linear printhead, the linear printhead extending in a cross-track direction and including an array of light sources for exposing a photosensitive medium, comprising:

a) providing digital image data for a test target including a plurality of alignment marks positioned at predefined cross-track positions;

b) printing the test target using the digital printing system to provide a printed test target;

c) using a digital image capture system to capture an image of the printed test target;

d) using a data processing system to automatically analyze the captured image to determine a measured position for each of the alignment marks;

e) comparing the measured positions for the alignment marks to reference positions to determine measured cross-track position errors;

f) determining a cross-track position correction function responsive to the measured position errors, wherein the cross-track position correction function specifies cross-track position corrections to be applied as a function of cross-track position;

g) storing a representation of the cross-track position correction function in a digital memory;

h) receiving digital image data for a digital image to be printed by the digital imaging system, wherein the digital image includes a plurality of image lines extending in the cross-track direction;

i) determining corrected image lines by resampling the image lines of the digital image responsive to the stored representation of the cross-track position correction function; and

j) printing the corrected image lines using the digital printing system to provide a printed image with reduced cross-track position errors;

wherein the cross-track position correction function is non-linear.

2. The method of claim 1 , wherein the digital printing system includes a plurality of color channels, wherein one of the color channels is designated to be a reference color channel and the other color channels are designated to be non-reference color channels, and wherein the reference positions for the alignment marks for the non-reference color channels correspond to measured positions of corresponding alignment marks printed with the reference color channel.

3. The method of claim 2 , wherein a predefined color channel is designated to be the reference color channel.

4. The method of claim 2 , wherein a cross-track line length is determined for each of the color channels, and wherein the color channel determined to have a longest cross-track line length is designated to be the reference color channel.

5. The method of claim 1 , wherein the reference positions correspond to ideal positions of the alignment marks.

6. The method of claim 1 , wherein the printed test target is on a print medium, and wherein the digital image capture system captures an image of the printed test target on the print medium.

7. The method of claim 1 , wherein the printed test target is on an imaging surface, the imaging surface being a surface of a photoconductor, a surface of an intermediate transfer member or a surface of a transport web, and wherein the digital image capture system captures an image of the printed test target on the imaging surface.

8. The method of claim 1 , wherein the plurality of alignment marks includes at least ten alignment marks.

9. The method of claim 1 , wherein the light sources of the linear printhead are LED light sources.

10. The method of claim 1 , wherein the resampling of the image lines of the digital image is also responsive to a resize factor.

11. The method of claim 1 , wherein the digital image capture system is a component of the digital printing system.

12. A method for correcting cross-track position errors in a digital printing system having a linear printhead, the linear printhead extending in a cross-track direction and including an array of light sources for exposing a photosensitive medium, comprising:

a) providing digital image data for a test target including a plurality of alignment marks positioned at predefined cross-track positions;

b) printing the test target using the digital printing system to provide a printed test target;

c) using a digital image capture system to capture an image of the printed test target;

d) using a data processing system to automatically analyze the captured image to determine a measured position for each of the alignment marks;

e) comparing the measured positions for the alignment marks to reference positions to determine measured cross-track position errors;

f) determining a cross-track position correction function responsive to the measured position errors, wherein the cross-track position correction function specifies cross-track position corrections to be applied as a function of cross-track position;

g) storing a representation of the cross-track position correction function in a digital memory;

h) receiving digital image data for a digital image to be printed by the digital imaging system, wherein the digital image includes a plurality of image lines extending in the cross-track direction;

i) determining corrected image lines by resampling the image lines of the digital image responsive to the stored representation of the cross-track position correction function; and

j) printing the corrected image lines using the digital printing system to provide a printed image with reduced cross-track position errors;

wherein the linear printhead includes a plurality of individual light source chips, each including a plurality of light sources, and wherein alignment marks are positioned in proximity to boundaries between adjacent light source chips.

13. A method for correcting cross-track position errors in a digital printing system having a linear printhead, the linear printhead extending in a cross-track direction and including an array of light sources for exposing a photosensitive medium, comprising:

a) providing digital image data for a test target including a plurality of alignment marks positioned at predefined cross-track positions;

b) printing the test target using the digital printing system to provide a printed test target;

c) using a digital image capture system to capture an image of the printed test target;

d) using a data processing system to automatically analyze the captured image to determine a measured position for each of the alignment marks;

e) comparing the measured positions for the alignment marks to reference positions to determine measured cross-track position errors;

f) determining a cross-track position correction function responsive to the measured position errors, wherein the cross-track position correction function specifies cross-track position corrections to be applied as a function of cross-track position;

g) storing a representation of the cross-track position correction function in a digital memory;

h) receiving digital image data for a digital image to be printed by the digital imaging system, wherein the digital image includes a plurality of image lines extending in the cross-track direction;

i) determining corrected image lines by resampling the image lines of the digital image responsive to the stored representation of the cross-track position correction function; and

j) printing the corrected image lines using the digital printing system to provide a printed image with reduced cross-track position errors;

wherein the cross-track position correction function is quantized to integer cross-track position corrections.

14. The method of claim 13 , wherein the representation of the cross-track position correction function stored in the digital memory includes the cross-track positions and transition direction of transitions in the quantized cross-track position correction function.

15. The method of claim 14 , wherein transitions in the quantized cross-track position correction function are shifted so that they do not align with cross-track positions corresponding to fine lines in the digital image.

16. The method of claim 14 , wherein a plurality of different quantized cross-track position correction functions are provided where the transitions are shifted to different cross-track positions, and wherein a user selects one of the quantized cross-track position correction functions wherein the transitions do not align with cross-track positions corresponding to fine lines in the digital image.

17. The method of claim 14 , further including automatically analyzing the digital image to identify the cross-track positions of fine lines in the digital image, and wherein any transitions in the quantized cross-track position correction function that align with the identified cross-track positions of fine lines are automatically shifted to different cross-track positions.